It was on Easter Sunday evening that Jesus appeared to his Apostles, gathered together in the Upper Room, where they had eaten the Last Supper. As they gaped and shrank back in a mixture of fear and dawning hope, Jesus spoke to them reassuringly.
Let St. John (20:19-23) tell it:
Jesus came and stood in the midst and said to them, 'Peace be to you!' And when he had said this he showed them his hands and his side. The disciples therefore rejoiced at the sight of the Lord. He therefore said to them again, 'Peace be to you! As the Father has sent Me, I also send you.' When he had said this, he breathed upon them, and said to them, 'Receive the Holy Spirit; whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained.'
To paraphrase our Lord's words in more modern terms, what he said was this:
As God, I have the power to forgive sin. I now entrust the use of that power to you. You will be My representatives. Whatever sins you forgive, I shall forgive. Whatever sins you do not forgive, I shall not forgive.
Jesus knew well that many of us would forget our brave baptismal promises and commit grave sins after our Baptism. He knew that many of us would lose the grace, the sharing-in-God's-own-life which came to us in Baptism.
Since God's mercy is infinite and unwearying, it seems inevitable that he would provide a second chance (and a third and a fourth and a hundredth if necessary) for those who might relapse into sin.